Coagulation Factor (F) VIII is mostly made in the liver. Its deficiency is the mot common inherited severe bleeding disorder. Present treatment involves intravenous infusions of FVIII. This therapy has drawbacks, and while efforts are underway to develop a form of somatic gene therapy, successful programs have yet to be achieved. The objective of this application is to develop an alternative approach, ectopically expressing FVIII in platelets. Preliminary studies show that transgenic mice expressing human (hu) FVIII driven by the murine (mu) Platelet Factor 4 (PF4) promoter can ameliorate the bleeding diathesis in FVIII-deficient (FVIII) mice. Specific aims are as follows: (1) Complete the analysis of the initial founder lines expressing huFVIII. These studies will further document the platelet-specific nature of the huFVIII, document its level of expression, and more fully define the degree of correction of the FVII/I- phenotype. (2) Improve the efficiency of the construct for huFVIII expression in megakaryocytes. This aim will try several construct variations to obtain higher levels of huFVIII expression platelets, and correlate levels of expression with phenotypic improvement. (3) Assess the efficacy of megakaryocyte expression of huFVIII in the context of a viral transfer system in a murine model. These studies will further test our model of whether huFVIII expression in hematopoietic stem cells is a viable approach for the treatment of Hemophilia A using retroviral constructs. These vectors will be tested on FVIII-mice recipients and the bleeding diathesis, the level of platelet huFVIII expression and the duration of correction will be determined. These studies will provide a useful pre-clinical evaluation of the proposed approach to genetic therapy for FVIII deficiency. We believe that ectopically expressed FVIII will be a viable way to treat FVIII deficiency, resulting in a longer FVIII half-life and in the delivery of FVIII in a concentrated fashion at the site of injury. We also believe that this approach may provide a source of "protected" FVIII that would be useful in patients with inhibitors. Finally, we believe that this approach may serve as a model for gene therapy to deliver other proteins in concentrated fashion to a site of injury.